Such a mechanism is described in the specification of U.S. Pat. No. 4,934,503, and constitutes a unitary module. In that patent specification, the driven member has at its inner periphery a flange or radial plate which extends radially towards the axis of the assembly and on which the actuating member (i.e. a clutch release bearing) is mounted by means of a special ring. Such an arrangement complicates the fitting operation, because the actuating member cannot be fitted by simply threading it axially on to the driven member of the control mechanism. The driven member surrounds the clutch release bearing, so that in some cases the size of the latter is undesirably restricted. In addition, the radial size of the whole arrangement is relatively large.